High Voltage Direct Current (HVDC) converters are mainly classified as Current Source Converters (CSC) and Voltage Source Converters (VSC). CSC converters are based on Line Commutated Converters (LCC) which contains thyristors. Thyristors have some inherent advantages such as being rugged, having low ON state losses, being easy to connect in series with each other, having a short time over-rating capability and having high power rating and being low cost. Moreover, thyristors are well evolved and commonly used components. However, LCC converters have a number of shortcomings: they may experience commutation failure, they require a strong alternating current (AC) grid, they do not support reactive power, have high harmonics, require polarity reversal for power reversal, leave a large footprints, are slow in control and unable to improve AC grid stability.
VSC converters are based on self commutating devices such as Insulated Gate Bipolar Transistors (IGBT), Integrated gate-Commutated Thyristors (IGCT), etc. This type of converters overcomes many shortcomings of LCC.
It has the following features: no commutation failure, it may be connected to a weak AC grid (also passive load), can support reactive power, has low harmonics, does not require polarity reversal for power reversal, is fast in control and able to improve AC grid stability.
However, VSC converters have the following disadvantages: higher ON state loss, higher switching loss, increased rating (due to fault current), frequent failure and higher cost.
Recently VSC employing multiple cells have also been used, where each cell provides a voltage contribution in order to form a waveform that is used to obtain an AC signal.
These converters, which are also termed modular multilevel converters, (M2LC), i.e. multi-cell VSC converters, have some further advantages. They may be provided without filters. Also di/dt and dv/dt problems can be avoided. However they also provide a larger footprint (cell capacitors).
In order to reduce the number of cells various types of director switch solutions have been proposed, where some are described in WO 2011/098117 and WO 2010/088969.
With regard to this it would be of interest to provide an improved voltage source converter, where the advantages of the CSC is obtained.